Fast fission nuclear reactors, cooled by liquid metal, comprise a core consisting of fuel arrays arranged side-by-side, and in contact via their lateral faces, in a vessel containing the liquid metal for cooling the reactor, which is generally liquid sodium. The liquid sodium for cooling the arrays, or primary sodium, is circulated by means of pumps. The circulating sodium passes through the core from bottom to top, in the longitudinal direction of the arrays, with the result that the exit from the core is in the upper part.
Each of the arrays constituting the core is itself formed of a set of long tubes or needles consisting of a canning material containing the fissile fuel materials or fertile materials.
In this type of reactor, rapid detection of the cracks which can appear in the canning material of some of the arrays, during the operation of the reactor, is very important. These breaks in cans can be detected by demonstrating the presence of fission products in liquid metal constituting the reactor cooling fluid. In fact, in the case of a break in a can, the fission products are released by the fuel material into the cooling fluid in contact with the external surface of the can.
The defective fuel arrays can be precisely located by removing samples of liquid metal coolant in the region of these arrays, near the exit from the core, i.e., in the upper part of the core, through which the heated liquid metal leaves the core.
Thus, a set of sampling pipes is arranged above the core, each of the pipes in this set being associated with one fuel array, and the end of these pipes which is opposite the sampling end arranged above the array is connected to means for identifying and locating the defective arrays by detecting fission products in the cooling fluid.
These identifying and locating means are grouped together to form one or more modules, referred to as can break location (CBL) modules, embedded in the slab covering the vessel above the reactor core.
These modules generally comprise selectors and measuring means, such as neutron counters, which make it possible successively to analyze the samples removed from each of the arrays, and to determine those arrays for which the cooling fluid contains fission products representing a leak in the array in question.
In the case of the fast fission nuclear reactors currently being constructed, the number of arrays in the core is generally of the order of 500, which requires the presence of the same number of sampling tubes connected to the selecting and measuring means.
It has therefore seemed preferable to use several modules, and the sampling pipes corresponding to the arrays located in a given region of the core and connected to each of these modules.
The locating modules comprise moving or fragile components, such as the sample selectors, the pumps for circulating the liquid metal, and the neutron detectors, which must remain functional throughout the operating period of the reactor. These modules can therefore be subject to breakdowns or shutdowns, with the result that the monitoring of a whole region of the core may be temporarily interrupted.
This is incompatible with continuous operation of the reactor under very good safety conditions.